Regeneration of used supported noble metal catalysts

ABSTRACT

A method for regenerating used supported noble metal catalysts, which method includes solvent cleaning the used catalyst by contact with a suitable organic liquid cleaning solvent such as alcohols, ketones and such to remove organic deposits from the catalyst, followed by drying and calcining at elevated temperature to remove any remaining organic deposits from the catalyst, then treating the catalyst with an organo-metallic complex forming agent having ionization constant pK 1  greater than about  2.5 , such as glycolic acid and the like. The organic-metallic complex forming agent acts to break down large clusters of noble metal particles such as palladium (Pd) and redistributes the metal particles on the catalyst support such as alumina (Al 2 O 3 ) in the same or other larger pores, so as to increase catalyst surface area and catalytic activity to provide a catalytic activity level at least 80% or even exceeding that of the fresh catalyst. This regeneration method is particularly useful for regenerating used supported palladium catalysts utilized for hydrogenation of ethyl anthraquinone (EAQ) for producing hydrogen peroxide (H 2 O 2 ) product.

BACKGROUND OF INVENTION

[0001] This invention pertains to regeneration of used supported noblemetal catalysts. It pertains particularly to regenerating used supportednoble metal catalysts such as palladium catalysts utilized forhydrogenation of ethyl anthraquinone (EAQ) for producing hydrogenperoxide (H₂O₂) product, so as to achieve catalyst activity levels nearor exceeding that of the fresh catalyst.

[0002] The conventional production of hydrogen peroxide product involvesa two-step process in which a hydrogen donor solvent ethyl anthraquinone(EAQ) is first hydrogenated and then oxidized with oxygen to form thehydrogen peroxide product. In some hydrogen peroxide manufacturingfacilities, the hydrogenation step is carried out in a fixed bed reactorutilizing a palladium/alumina or similar catalyst. A typical suchcatalyst may contain 0.28 to 0.33 wt % palladium on a large pore aluminasupport. The useful life expectancy of the catalyst is about two years,after which its activity drops to about 30% of its original (freshcatalyst activity) condition or level. It is believed that this catalystdeactivation is caused by deposition of high molecular weight organicmaterials formed from the polymerization of EAQ on the active sites ofthe catalyst, and/or by gradual agglomeration of the palladium to largerparticles or clusters on the catalyst. Such spent palladium/aluminacatalysts are presently being regenerated using a simple “wash-burn”procedure, in which the catalyst is first extracted with an organicsolvent to remove any soluble material deposits, and then subjected to acontrolled carbon burn-out step at about 850° F temperature in air. Suchhigh temperature regeneration treatment may also promote undesirableagglomeration of the palladium to larger particles on the catalystsupport. Thus, it is difficult to successfully regenerate the usedcatalyst back to near 100% of its original activity. In fact, thissimple wash-burn procedure can usually restore the used catalyst to onlyabout 70% of its original or fresh activity level. Such “wash-burn”catalyst regeneration procedures have been disclosed by various U.S. andforeign patents. For example, U.S. Pat. No. 4,148,750 to Pine disclosesa process for redispersal of noble metals on used supportedzeolite-containing catalysts. U.S. Pat. Nos. 4,478,705 and 4,592,826 toGanguli disclose method steps for regenerating used catalysts by diluteacid treatment to remove undesired metal deposits followed by carbonburn-off at increased temperature levels. Also, U.S. Pat. No. 5,188,996to Huang et al discloses redispersion of noble metal such as platinum onlow acidity support such as silica by contacting with chlorine andoxygen at low pressures.

[0003] It is desired to improve economics of the hydrogen peroxideproduction process by increasing the activity and service life of thepalladium/alumina catalyst, as well as that of other similar supportednoble metal catalysts. Based on an understanding at molecular level ofthe apparent catalyst reaction and deactivation mechanism, the surfacestructure of the catalyst support material, and the exposition ofpalladium crystal clusters thereon, an effective procedure has beendeveloped for regenerating and enhancing used palladium/alumina catalystto an activity level significantly higher (90% or more) than thatachieved by the current “wash-burn” procedure.

SUMMARY OF INVENTION

[0004] This invention provides a regeneration method and procedure forregenerating and enhancing used supported noble metal catalysts. For thefresh catalyst, the support material is alumina having surface area of20-600 m²/gm and pore diameters within the range of 50-600 Angstroms,with 50-500 m²/gm and 100-400 Angstroms being preferred. The noblemetals include palladium (Pd), platinum (Pt), gold (Au), iridium (Ir),osnium (Os), rhodium (Rh), or ruthenium (Ru), or combinations thereof,with palladium usually being preferred. The invention is particularlyuseful for regenerating and enhancing used supported palladium (Pd)catalyst, such as utilized for hydrogenation of ethyl-anthraquinone(EAQ) for producing hydrogen peroxide (H₂O₂) product.

[0005] The used catalyst regeneration method of this invention includesthe following three basic steps:

[0006] (1) Cleaning the used supported noble metal catalyst by solventextraction for removal of process contaminants and adsorbed chemicalsfrom the used catalyst by contact with suitable organic solvent(s);

[0007] (2) Drying and calcining the cleaned used catalyst to remove anypolymer deposits remaining on the catalyst;

[0008] (3) Contacting the cleaned and calcined catalyst with a suitableorganic treating agent selected for forming an organo-metallic complexfor breaking down large noble metal agglomerates on the used catalyst tosmaller metal particles, and redistributing the smaller noble metal(s)particles on the catalyst support. Suitable catalyst treating agentsshould have an ionization constant pK₁ greater than about 2.5.

[0009] The organic solvents suitable for the used catalyst cleaningmethod step (1) by solvent extraction of the used supported noble metalcatalyst can be alcohols such as methanol, amines, ketones, or similarorganic compounds utilized at cleaning conditions of 0-200° C.temperature and 1-50 atm. pressure for 2-8 hours. Preferred solventcleaning conditions are 10-100° C. temperature and 1-20 atm. pressurefor 4-6 hours. Suitable catalyst drying and calcining conditions for themethod step (2) are heating the catalyst in air at 100-120° C. for 1-8hours for the drying, then further heating it in air at 200-600° C.(392-1112° F.) temperature for 1-24 hours for the calcining step.Calcining the dried catalyst at lower temperatures and longer timeperiods within these ranges is usually preferred for economic reasons.

[0010] The organo-metallic complex forming chemical treating agentssuitable for the noble metal redistribution method step (3) on thecleaned noble metal catalysts are chemical compounds having carbon atomsnot exceeding about 20 and molecular weight not exceeding about 300. Thetreating agent should also have an ionization constant pK₁, greater thanabout 2.5, as defined by the following equation and its transformation:RCOOH + H₂O ⇆ RCOO⁻ + H₃O⁺$K_{1} = \frac{\left\lbrack {RCOO}^{-} \right\rbrack \left\lbrack {H_{3}O^{+}} \right\rbrack}{\lbrack{RCOOH}\rbrack}$

[0011] K₁ should be less than 1×10^(−2.5), and pK₁ should be greaterthan 2.5.

[0012] Some examples of organo-metallic complex forming chemicaltreating agents and their corresponding ionization constants pK₁ are asfollows: Treating Agent pK₁ Oxalic Acid 1.27 EDTA 2.01 Citric Acid 3.13Glycolic Acid 3.63 Succinic Acid 4.21 Glycine 9.78 Salicylic Acid 13.12

[0013] Oxalic acid having pK₁ of 1.27 and ethylene diaminetetraaceticacid (EDTA) having pK₁ of 2.01 are outside this desired range, and arethereby not suitable for providing desired organo-metallic complexes forthis invention. Also, EDTA has been shown to remove aluminum fromzeolites by a chelation effect which can thereby render zeolite supportsineffective by deactivating the support. Useful reaction conditions forforming the organo-metallic complexes and for redistributing the noblemetal particles are 10-500° C. temperature and 1-10 atm. pressure for1-8 hours, with 20-450° C. temperature and 1-5 atm. pressure for 2-6hours being preferred. For best results, the organic treating agentshould preferably be maintained in its liquid phase, however aliquid/vapor phase mixture having only a small portion vapor may beutilized.

[0014] By utilizing the catalyst regeneration method and procedureaccording to this invention, it is proved experimentally that usedpalladium (Pd) catalyst supported on alumina can be better cleaned forremoval of the process contaminants and polymer deposits, and thusexpose more catalyst surface and active Pd sites to a process reactant.Catalyst activity tests have also shown that used supported Pd catalystsregenerated by the method of this invention have their activitysignificantly increased to at least about 80% and preferably up to93%-103% of fresh catalyst activity level, compared to only about 70% offresh catalyst activity after being regenerated by known traditional“wash-burn” regeneration procedures. This regeneration method isparticularly useful for used supported catalysts containing 0.2-0.4 wt.% palladium deposited on a support of alumina or silica, as utilized forproducing hydrogen peroxide (H₂O₂) by hydrogenation ofethyl-anthraquinone (EAQ).

[0015] Advantages provided by the catalyst regeneration method andprocedure of this invention include its ability to not only effectivelyremove contaminants and organic deposits from the used noble metalcatalyst, but also to break apart and redistribute the active noblemetal molecules such as palladium in the pores of the catalyst support.This new catalyst regeneration method and procedure not only cleans thecontaminated catalyst surface, but also improves the exposition anddistribution of the noble metal(s) such as palladium on the catalystsupport. The regeneration procedure can restore catalyst activity to100% or more of the fresh catalyst standard, and the resulting molarselectivity ratio of desired product to side products is 190:1, which isa better molar selectivity ratio than that achieved for fresh catalyst(150:1). This catalyst regeneration procedure and method is considereduseful for regenerating supported noble metal catalysts containing othernoble metals instead or in addition to palladium. After such usedcatalyst regeneration, the process reactant have improved contact withthe catalyst active metal(s) particles and thereby enhance the activityand product selectivity of the catalyst.

BRIEF DESCRIPTION OF DRAWINGS

[0016] FIGS. 1(a)-1(d) show schematic illustrations of typicaldeposition of noble metal(s) such as palladium in the pores of thesupport for fresh, spent and regenerated supported palladium catalysts,respectively.

DESCRIPTION OF INVENTION

[0017] The catalyst overall regeneration method and procedure developedfor the used supported noble metal catalysts, such as palladium (Pd)catalyst on alumina support, includes the following specific steps:

[0018] 1. Cleaning the used supported Pd catalyst having organicdeposits by contact with a selected liquid cleaning solvent such asmethanol at 0-200° C. temperature and 1-50 atm. pressure to dissolve andsubstantially remove the organic deposits from the catalyst.

[0019] 2. Drying the used catalyst at 100-120° C. temperature for 1-8hours to remove the cleaning solvent from the catalyst.

[0020] 3. Calcining the cleaned catalyst in air at 200-600° C.temperature for 1-24 hours to remove any remaining organic deposits fromthe catalyst.

[0021] 4. Adsorbing a suitable organic treating agent selected forforming an organo-metallic complex on the catalyst, and breaking downlarge Pd particle clusters and relocating or redistributing theresulting smaller palladium particles in pores of the support materialby contact with the organo-metallic complex forming agent liquid andvapor, such as glycolic acid having ionization constant pK₁ of 3.63.Suitable treating agent contacting conditions are 10-500° C. temperatureand 1-10 atm. pressure for 2-8 hours.

[0022] When utilizing the supported palladium on alumina catalyst forproducing hydrogen peroxide product from ethyl anthraquinone (EAQ), thecritical diameter of intermediate EAQ:H₂ dimer molecules is about 120 Å.Therefore, the ideal pore diameter of the alumina support should be atleast about 1.5 and preferably about 2.0 times that of the dimermolecules, i.e. at least about 180 Å and preferably at least 240 Å, soas to allow free movement of the reactant dimer from the adsorbed siteon the catalyst. During the used catalyst regeneration, it is desirableto break up Pd particle clusters from pores having diameter smaller thanabout 180 Å, and relocate the resulting smaller palladium particles intopores having diameters larger than about 180 Å. It is also desirable toavoid depositing the Pd particles into the catalyst pores having a sizesmaller than about 180 Å. Thus, for this used palladium catalystregeneration method, it is desirable to relocate the Pd particles fromthe pores smaller than about 180 Å into those pores larger than 180 Å,and preferably larger than 240 Å.

[0023] The four main reasons for the used supported Pd catalystdeactivation during hydrogenation of ethyl anthraquinone (EAQ) toproduce hydrogen peroxide are: (1) contamination of the Pd catalyst bypoisoning chemicals in the process feed or solvent; (2) coke depositionor large polymer molecule formation blocking the active Pd sites; (3) Pdparticles agglomeration to form clusters; and (4) Pd leaching from thecatalyst. The first three reasons for catalyst deactivation are at leastpartially reversible by regeneration, but the Pd loss by leaching isirreversible. After the palladium is lost from the catalyst, it is notpossible to restore the catalytic activity to near its initial ororiginal level.

[0024] Experimental data have indicated some catalyst deactivation dueto the first three listed reasons. Theoretical understandings of thereaction mechanism, catalyst structure and deactivation provide thebasis for designing this improved catalyst regeneration method andprocedure for used or spent supported noble metal catalysts, such assupported palladium catalysts. In order to regenerate the spent catalystto a highly active and product selective state, the regeneration methodmust achieve the following critical requirements:

[0025] 1) solvent clean the used Pd catalyst surface to substantiallyremove its contaminants and organic deposits,

[0026] 2) breakdown the large Pd particles clusters on the catalyst tosmaller particles, and

[0027] 3) relocate the smaller Pd particles from small pores to thosehaving a diameter larger than about 180 Å.

[0028] Experimental results have shown that the first two goals wereachieved, and it is believed that the third goal also was achieved, asthe regenerated catalyst activity and selectivity results indicate thatthe Pd was redistributed or relocated to a desirable state on thecatalyst support. In fact, when a suitable organo-metallic complexforming and redistributing agent such as glycolic acid is utilized, thefollowing effects on the Pd particles are believed to occur: (1) Thereaction between Pd clusters and glycolic acid treating agent breaksdown the large Pd clusters to the smaller clusters and particles. (2)The glycolic acid agent helps to intercalate in between the Pdparticles, thus allowing a more even distribution of these metalparticles on the support. (3) The glycolic acid treating agent can alsoenter the pores smaller than 180 Å and react with Pd. When morePd-glycolate complexes are formed, the pores are too small to hold allthe complexes, and the Pd-glycolate material is sequentially squeezedout of pores smaller than 180 Å. After these Pd glycolates move into thelarger pores, several Pd-glycolates will combine together by hydrogenbonding to form a large Pd-glycolate cluster, and this effect willprevent the Pd from depositing into the pores smaller than 180 Å.

[0029] The surface of typical fresh and spent supported palladium (Pd)catalysts, and catalyst regenerated according to this invention, areshown schematically in FIGS. 1(a)-1(d). As seen in FIG. 1(a), thesurface of fresh catalyst is clean, and the Pd particles are depositedrandomly in both small and large pores due to the catalyst traditionalnon-particle-size control preparation procedure. For the used or spentsupported noble metal catalyst (FIG. 1b), due to the long termexposition of catalyst under the reaction conditions the Pd particleshave agglomerated to form larger clusters, thereby at least partiallyblocking the catalyst small pores. The organic deposits are also formedon the Pd surface and alumina support, and can block more small pores.These effects result in a significant decrease of catalyst activity,surface area, and percentage of pores smaller than about 200 Åexposition.

[0030] Traditional catalyst regeneration methods by heating to about450° C. (842° F.) temperature for several hours can clean the catalystsurface (Fig lc). However, the sintering of Pd during such catalystheating forms larger particles which block the entrance of many smallpores of the support. The low surface exposition of large Pd particlesresults in limited access of reactants to the active Pd, thus leading toa less active catalyst than the fresh catalyst standard.

[0031] This invention will be further described by reference to thefollowing example, which should not be construed as limiting the scopeof the invention.

EXAMPLE

[0032] Samples of used supported palladium (Pd) on alumina catalyst,obtained from extended operations for hydrogenation of ethylanthraquinone (EAQ) for producing hydrogen peroxide (H₂O₂) product, wereregenerated utilizing the method of this invention. The used catalystcontained 0.2 -0.4 wt. % palladium on alumina support. The used catalystwas first contacted with methanol solvent at 25° C. and ambient pressurefor 3.3 hours, then replaced with new methanol solvent three times witheach time for 30 minutes (0.5 hour). Then the washed catalyst was driedin air at 110° C. for 2 hours, then calcined in air at 400° C. for 4hours. The calcined catalyst was then treated with glycolic acid agentat 400° C. and ambient pressure for 3 hours. The results obtained withthe used catalyst that was regenerated by this procedure are shown inTable 1, and are depicted schematically in FIG. 1(d). TABLE 1Regeneration of Used Supported Palladium Catalyst Fresh Used Wash-BurnRegenerated Catalyst Surface Area, m²/g 82.6 80 76.7 88.1 Pores Diameter<240Å, % 9.5 13.0 Desired Product/Side Product 150:1 190.5 Molar RatioCatalyst Activity Relative to 100 ˜30 70 90-103 Fresh Catalyst, %

[0033] From the above results, it is noted that after the catalyst wascleaned by methanol solvent and re-generated by contact with glycolicacid treating agent, its surface area increased to 88.1 m²/g, which isdesirably greater than that of the fresh catalyst standard (82.6 m²/g).The percentage of pores <240 Å also increased to 13.0% from 9.5% for thespent catalyst. These results indicate that the catalyst regenerationmethod of this invention not only cleans all the organic deposits fromthe used catalyst, but also clears the blockage of small pores in thesupport, which means that the large Pd particles on the used catalystwere broken down to smaller particles for the regenerated catalyst.Also, the increased surface area indicates that the Pd particle size issmaller than that of the fresh standard, and the particles are depositedmainly in the larger pores, otherwise the catalyst surface area wouldnot increase significantly. This explanation is fully supported by thecatalyst activity and selectivity test results.

[0034] Used catalyst regenerated by the method of the present inventionhas an activity close to or even exceeding 100% of the fresh catalyststandard. Because the spent catalyst had been used for years, someattrition of Pd from the support is unavoidable, but this new catalystregeneration procedure restored the activity to near 100% or more of thefresh standard activity. This result indicates that for the regeneratedcatalyst the Pd active metal is being used more efficiently, e.g. the Pdis exposed on the catalyst surface in smaller particle size and atlocations which are easy for process reactants to reach, i.e in poreslarger than about 240 Å diameter.

[0035] The molar ratio of desired H₂0₂ product to side product after thecatalyst regeneration (190:1) also exceeded that for the fresh catalyststandard (150:1). The high selectivity is apparently an effect of Pddeposited in pores >240 Å. As discussed above, to avoid overhydrogenation of EAQ:H₂ dimer and formation of the undesired productEAQ:H₄, one must try to minimize the time during which EAQ:H₂ remains atthe adsorbed site, and this intermediate material must be removed assoon as possible. The critical diameter of intermediate EAQ:H₂ dimer isabout 120 Å. Ideally, it should be avoided to deposit Pd into thecatalyst pores that have a diameter smaller than 240 Å, and in which thefree movement of the dimer is restricted and excess hydrogenation isunavoidable.

[0036] Although this invention has been described broadly and also interms of specific preferred embodiments, it will be understood thatmodifications and variations may be made to the invention as definedwithin the scope of the following claims.

We claim:
 1. A method for regenerating used supported noble metalcatalyst, comprising the steps of: (a) providing a used noble metalcatalyst having organic deposits contained on the catalyst, saidcatalyst including alumina support having initial surface area of 20-600m²/g and containing palladium (Pd), platinum (Pt), gold (Au), iridium(Ir), osnium (Os), rhodium (Rh), or ruthenium (Ru) on the supportmaterial; (b) cleaning said used catalyst by contacting it with aselected organic liquid cleaning solvent at 0-200° C. temperature and1-50 atm. pressure for sufficient time to substantially dissolve organicdeposits from the catalyst; (c) drying said used catalyst and removingthe organic liquid cleaning solvent; (d) calcining said dried catalystat 200-600° C. temperature for 1-24 hours and removing any remainingorganic deposits; and (e) adsorbing on said dried catalyst anorgano-metallic complex forming agent while breaking down metal particleclusters of the noble metal and relocating the resulting smaller metalparticles on the support, by contacting the catalyst with theorgano-metallic complex forming agent at contacting conditions of10-500° C. temperature and 1-10 atm. pressure, and thereby regeneratingthe catalyst activity level to at least 80% of its fresh catalystactivity, said agent having up to 20 carbon atoms, molecular weight notexceeding 300, and having ionization constant pK₁, exceeding about 2.5.2. The catalyst regeneration method of claim 1, wherein said usedcatalyst contains 0.2-0.4 wt. % palladium (Pd) on alumina support havingpore diameter range of 50-600 Å.
 3. The catalyst regeneration method ofclaim 1, wherein the organic cleaning solvent is liquid methanol at10-100° C. temperature and 1-20 atm. pressure for 2-8 hours.
 4. Thecatalyst regeneration method of claim 1, wherein the drying step occursby heating in air at 100-120° C. temperature for 1-8 hours.
 5. Thecatalyst regeneration method of claim 1, wherein the calcining stepoccurs in air at conditions of 250-500° C. (482-932° F.) temperature andambient pressure for 2-10 hours.
 6. The catalyst regeneration method ofclaim 1, wherein the organo-metallic complex forming agent is glycolicacid utilized at 20-400° C. temperature and 1-5 atm. pressure for 1-8hours.
 7. The catalyst regeneration method of claim 2, wherein theregenerated catalyst activity is restored to 90-103% of fresh catalystactivity.
 8. The catalyst regeneration method of claim 2, includingutilizing the regenerated supported palladium (Pd) catalyst forhydrogenation of ethyl anthroquinone (EAQ) for producing hydrogenperoxide (H₂O₂) product.
 9. A regeneration method for used supportedpalladium catalyst, comprising the steps of: (a) providing a usedpalladium (Pd) catalyst on alumina support having organic deposits onthe catalyst, said catalyst having initial surface area of 50-500 m²/g.(b) cleaning said used supported palladium (Pd) catalyst by soaking itin liquid methanol cleaning solvent at 10-100° C. temperature and 1-20atm. pressure for 2-8 hours and dissolving organic deposits from thecatalyst; (c) drying said used catalyst at 100-120° C. temperature andremoving the methanol cleaning solvent; (d) calcining said driedcatalyst in air at 350-500° C. temperature for 2-10 hours and removingany remaining organic deposits from the catalyst; and (e) adsorbingglycolic acid treating agent on the dried and calcined catalyst whilebreaking down large palladium particle clusters and relocating thepalladium particles on the catalyst support by action of the glycolicacid treating agent utilized at 20-400° C. temperature and 1-5 atm.pressure for 1-8 hours, and thereby regenerating the catalyst activitylevel to 90-103% of fresh catalyst activity.
 10. The catalystregeneration method of claim 9, including utilizing the regeneratedsupported palladium (Pd) catalyst for hydrogenation of ethylanthraquinone (EAQ) for producing hydrogen peroxide (H₂O₂) product.